The state-of-charge (SOC) of a high-voltage battery, such as the type used in hybrid electric vehicles (HEVs) for vehicle propulsion, typically refers to the amount of charge or energy left in the high-voltage battery and is usually expressed in terms of a percentage. Those skilled in the art will appreciate that it may be desirable for certain types of high-voltage batteries, like those based on lithium-ion chemistries, to be maintained in a certain SOC range. Overcharging and/or undercharging high-voltage batteries such that the SOC goes beyond the boundaries or limits of the corresponding SOC range may result in diminished battery life, battery performance, vehicle fuel economy, or other undesirable consequences.
High-voltage batteries usually need to preserve enough charge or energy so that they can perform certain duties—like starting the vehicle when requested—and the temperature of the battery can affect this. For instance, if it is very cold out (e.g., less than −13° C.), a high-voltage battery based on a lithium-ion chemistry may not have enough power to start a hybrid electric vehicle (HEV). The internal resistance of lithium-ion battery cells usually goes up as the battery temperature goes down, which results in a reduction of available cell current or amperage. High-voltage battery packs, like the ones typically used in HEVs for vehicle propulsion, usually include a number of cells connected in series; thus, the diminished amperage due to extremely cold temperatures may have a compounded affect, as each of the battery cells experiences an amperage reduction. Therefore, it may be necessary to manage the state-of-charge (SOC) in the high-voltage battery to account for this and/or other phenomenon.